As is known, in calipers for disc brakes, opposite pads are pushed by means of at least one piston adapted to slide in a respective seat in the body of the caliper, against opposite braking surfaces of a braking band of a brake disc which can be associated with the caliper to stop the disc brake.
Electromechanical brake calipers comprise an electromechanical actuator having an electric motor and a device for converting the motion from rotational motion output from the motor to translational motion of the piston.
For example, the motion conversion device comprises a handling screw, for example a recirculating screw, rotated by the motor, in which the screw is housed in the body of the piston so that a rotation of the screw corresponds with a translation of the piston.
By operating the electric motor in rotation in one direction of rotation or in the opposite direction of rotation, a translation moving the piston close to or away from the braking surfaces of the disc, respectively, is obtained.
Such electromechanical brake calipers may provide both the service braking and the parking braking.
In the case of service braking, such calipers need to ensure stability of the vehicle, high electromechanical performance, short response times and low inertias.
In the case of parking braking, once the required clamping load is reached, they have the need to ensure the braking force is maintained over time.
With reference to parking braking, Patent EP1032773B1 entitled to Continental Teves AG discloses a parking braking device which uses a ratchet that locks an element of a mechanical drive once the actuator is no longer electrically powered.
Such a device has the disadvantage that it is to be constantly powered during use to avoid the brake from locking and the disadvantage that it is instantly blocked as soon as the supply voltage of the device falls or is removed. Thus, in case of breakdown, for example a breakdown of the actuator, demagnetization, excessive heat, mechanical fatigue, the instantaneous locking of the brake would have a negative impact on the stability of the vehicle and on the safety of the driver.
Patent EP1218646B1 entitled to Continental Teves AG and INA Walzlager Schaeffler discloses an electromechanically-operated disc brake comprising a post and a locking element with a spherical or roller shape, for allowing or preventing the rotation of a drive wheel. The locking of the mechanical drive occurs by means of the operation of a solenoid electromagnetic actuator which, by translating the post towards the wheel, prevents the passage of an engagement sphere, thus locking the transmission and preventing a removal of a parking braking action.
This device has the disadvantage that the breakdown of the actuator would instantaneously cause an unwanted operation, thus causing the instability of the vehicle or the loss of parking of the vehicle.
Moreover, Patent EP1985883 entitled to Hitachi Ltd discloses an electric disc brake comprising a caliper body and a locking mechanism in which the locking mechanism moves with a thrust element of the pad and consists of a movable element having a plurality of teeth with a predetermined pitch in a direction of movement, an engagement element having an engagement portion which can be engaged with the teeth of the movable element, an actuator which guides the engagement element in such a direction that the engagement portion engages with the teeth of the movable element and operating means for operating the engaged element in a direction in which the engagement portion separates from the teeth of the movable element.
This device has the disadvantage that, by starting from the parking condition already applied, the reapplication of the load occurs only if the engagement element reaches at least the tooth adjacent to the already engaged tooth of the movable element. Should the motor not cover the entire angular sector between two teeth and be restricted to reaching a first intermediate point, this would result in the involuntary removal of the parking load, because the engagement element would come out of the movable element, thus releasing the braking load. Thus, an involuntary removal of the parking braking action could occur.
Moreover, this device has the disadvantage that the engagement element is constantly energized by an energizing means, for example a compression spring, in such a direction that such an engagement element is moved away from the teeth of the wheel. If such an energizing means should lose efficiency during the service, an involuntary application of the mechanism could occur.
Therefore, the need is strongly felt for a locking device of an electromagnetically-operated brake caliper capable of maximizing operation reliability and safety, while ensuring both the stability of the vehicle during the service braking and that the braking force is maintained during the parking braking.